Better handle a player not playing on either team in the game

Logging an error is not enough, if you let this player make changes
to the database! And since we're checking that, might as well not
update the things they shouldn't be seeing anyway
Improve consistency: all objects associated with a particular user should have a get_full_name method
A user not playing the game should not see the RSVP form
Use GameTeam objects when talking about a team playing a game

There is a reason these objects exist: they provide extra information
that applies to this team only in this game. Thus, if you are talking
about a team in a specific game, you should be dealing with a
GameTeam, not a Team
Make GameTeam tests reusable
Add comment objects to the database
Align field names across code
Delete code left in by accident

This code was moved into the function check_comments. Unfortunately,
it was also left in the original site.
Add functions for adding, retrieving, and displaying comments
Actually add and display comments